The present invention relates to a hybrid digital radio-relay system which has a transmitting terminal station, at least one repeater station, and a receiving terminal station, in particular, relates to such a system in which at least one repeater station is a non-regenerative repeater station which does not regenerate a digital signal, and distortion generated during relay sections is compensated or equalized in a receiving terminal station and/or a regenerative repeater station. A hybrid system in the present invention means that both a non-regenerative repeater station and a regenerative repeater station (or a regenerative receiving terminal station) are used.
Because of the latest development of a digital radio-relay system, the cost, the consumed electric power, and the maintenance time of the system have been increased. The main reason of those problems is that a conventional digital radio-relay system uses a regenerative repeater station which regenerates a digital signal in each repeater station, so that distortion and/or symbol error is equalized in each repeater station, and excellent communication quality is obtained.
However, a regenerative repeater station must have demodulators and modulators, and an equalizer, and therefore, it has the disadvantage that the structure of a repeater station is complicated.
Therefore, a non-regenerative repeater station which does not regenerate a digital signal in each repeater station and all the distortion is equalized in a receiving terminal station is promising. As fading in a relay section occurs at random, and the probability that fading occurs in a plurality of relay sections at the same time is very small, a non-regenerative relay system which equalizes at a receiving terminal station may have the similar transmission quality to that of a regenerative relay system which equalizes in each repeater station.
FIG. 1 shows a block diagram of a prior regenerative digital radio-relay system, which uses a multi-carrier system, having three carriers in the embodiment.
It is assumed in the embodiments that there are three sub-system signals (sys.1, sys.2 and sys.3) each having three carriers (A, B and C) on both a go-channel and a return-channel.
The transmitting terminal station 10 has three transmitter sub-units 12-1, 12-2 and 12-3, relating to a sub-system signal, and each transmitter sub-unit has three modulators 14-a, 14-b, and 14-c for modulating carriers A, B and C, respectively, and a transmitter 16 for frequency conversion to radio frequency f.sub.1 and high power amplification. The outputs of the transmitter sub-units are combined in the band splitter filter 17, and transmitted towards a repeater station through an antenna 18.
A regenerative repeater station 20 has a pair of antennas 21-1 and 21-2 to receive radio frequency f.sub.1, a band splitting filter 22 for separating each sub-system signals, three repeater sub-units 24-1, 24-2 and 24-3, another band splitting filter 23 for combining outputs of the repeater sub-units, and an antenna 40 which transmits signal towards a next repeater station or a receiving terminal station.
Each repeater sub-unit (24-1, 24-2, 24-3) has a receiver 26 for converting radio frequency to intermediate frequency (IF), a diversity combiner 28 for combining two received signals based upon conventional diversity process, an auto-gain controller 30 for amplifying received signal, three demodulators 32-a, 32-b and 32-c relating to three carriers for demodulating signals, three transversal equalizers 34-a, 34-b and 34-c for equalizing waveform distortion added to demodulated signals, three modulators 36-a, 36-b and 36-c for modulating signals, and a transmitter 38 for frequency conversion from IF frequency to radio frequency f.sub.2. The outputs of the sub-units are combined in the band splitting filter 23, and are transmitted to the next repeater station or a receiving terminal station through an antenna 40.
A receiving terminal station 50 has a pair of antennas 51-1 and 51-2 to receive radio signal, a band splitting filter 54 for separating sub-system signals, and three receiver sub-units 52-1, 52-2 and 52-3. Each receiver sub-unit has a receiver 56 for converting radio frequency f.sub.2 to intermediate frequency, a diversity combiner 58, an auto-gain controller 60, three demodulators 62-a, 62-b and 62-c, and three transversal equalizers 64-a, 64-b and 64-c.
FIG. 2 shows frequency allocation in a repeater station of FIG. 1. The received radio signal of frequency f.sub.1 has three sub-system signals sys.1, sys.2 and sys.3, each having three carriers (A, B and C et al).
In the frequency conversion from IF frequency to radio frequency or radio frequency to IF frequency, the transmitter 16 in the transmitting terminal station 10, and the receiver 26 in the repeater station 20 take a lower heterodyne in the frequency conversion, and the transmitter 38 in the repeater station 20 and the receiver 56 in the receiving terminal station 50 take an upper heterodyne.
The frequency conversion from radio frequency to IF frequency and vice versa is to mix the signal with a local frequency. It should be noted that two side bands are obtained in the mixing process, and one of the side bands are taken.
An upper heterodyne is defined so that the local frequency is higher than the selected side band signal.
A lower heterodyne is defined so that the local frequency is lower than the selected side band signal.
In the embodiment of FIG. 2, a lower heterodyne is taken and three sub-system signals (sys1; A,B,C), (sys2; D,E,F) and (sys3; G,H,I) each having three carriers (A, B, C et al) are obtained. f.sub.A, f.sub.B and f.sub.C are carrier frequencies of each carriers.
The transmitter 38 converts the IF frequency to the radio frequency f.sub.2. An upper heterodyne is taken in this case, so that the local frequency for frequency conversion is allocated within an assigned frequency band. In other words, when a receiver takes a lower heterodyne, a transmitter takes a higher heterodyne, and when a receiver takes an upper heterodyne, a transmitter takes a lower heterodyne. Therefore, the frequency allocation of three carriers in each sub-system signal in the transmitted signal is opposite to those of the received signal. It should be noted for instance that the sub-system signal sys.1 has the allocation A, B and C in frequency f.sub.1, on the other hand, it has the allocation C, B and A in frequency f.sub.2.
A regenerative repeater station regenerates a digital signal, and therefore, all the distortion in the propagation path is completely compensated in each repeater station.
However, when a non-regenerative repeater station is used, distortion and/or interference is not compensated in a repeater station. This is explained in accordance with FIGS. 3 and 4.
FIG. 3 shows a prior non-regenerative repeater station 80, which has a pair of antennas 80-1 and 80-2 for space diversity, a band splitting filter 82 for separating sub-system signals, three repeater sub-units 86-1, 86-2 and 86-3, another band splitter filter 84 for combining outputs of three sub-units, and an antenna 99.
Each sub-unit has a receiver 88 for converting received radio frequency to IF frequency, a diversity combiner 90, an auto-gain controller 92 for amplifying IF frequency signal, and a transmitter 98 which converts IF frequency to radio frequency.
The auto-gain controller 92 has a hybrid circuit (H) for separating three carriers, three bandpass filters each having center frequency F.sub.1, F.sub.2, and F.sub.3 for taking only one carrier (A, B, C et al), three auto-gain controllers A.sub.1 A.sub.2, A.sub.3 for amplifying carriers, and an adder 96 for combining three carriers.
It should be noted that a bandpass filter is not ideal, but has a wider pass band than that of a carrier, therefore, a part of the adjacent carrier leaks into the desired carrier which passes the bandpass filter. In FIG. 4, (1) shows that the carrier A is accompanied by undesired (b) which is a part of the adjacent carrier B. Similarly, (2) shows that the carrier B has a part of the adjacent carriers A and C. Similarly, the carriers A through I accompany a part of undesired carriers as shown in (1) through (9) in FIG. 4, because of non-ideal characteristics of a bandpass filter.
When three carriers are combined in the adder 96, the frequency allocation of each carriers is shown in (10), (11) and (12) in FIG. 4.
Then, when three sub-system signals (10), (11) and (12) are frequency converted from IF frequency to radio frequency, the frequency allocation is shown in (13) in FIG. 4. It should be noted in (13) that undesired noise (g) which is a part of the carrier G is included in the carrier A, and undesired noise (c) is included in the carrier I. Those undesired noises can not be compensated by an equalizer in a receiving terminal station, since A and G are different signals from each other. On the other hand, the carriers B and C, et al are compensated by an equalizer, since the leak noise (d) at the side of (C) is removed by a roll-off filter in a demodulator, and the leak noises (c) in (C), and (b) in (B) are combined in-phase as those signals as they are the same signals as one another.
It should be noted that the frequency allocation (13) in FIG. 4 shows the case of only one relay section. When many relay sections are used, the interference is complicated, and many unequalizable interferences are generated.
The leaked noise including undesired interference (g) and (c) in FIG. 4 is called Self-Interference caused by Passing Adjacent Channels (S-IPAC) in the present specification.
Because of the unequalizable interferences, a non-regenerative radio-relay system has not been used.